Steel sheet and manufacturing method thereof

Abstract

In the present invention, provided is a steel sheet having a predetermined chemical composition and a metallographic structure, in which A/B, which is a ratio of a length A of an interface between epitaxial ferrite and ferrite to a length B of an interface between the epitaxial ferrite and martensite in a cross section that is along a rolling direction and perpendicular to a surface of the steel sheet at a position of ¼ of a sheet thickness from the surface of the steel sheet is more than 1.5, a ratio of a major axis to a minor axis of the martensite is 5.0 or more, and a tensile strength is 980 MPa or more.

The invention claimed is: 
     
       1. A steel sheet comprising, as a chemical composition, in mass %,
 C: 0.050% or more and 0.500% or less, 
 Si: 0.01% or more and 2.50% or less, 
 Mn+Cr: 1.20% or more and 4.00% or less, 
 Al: 0.10% or more and 2.00% or less, 
 P: 0% or more and 0.100% or less, 
 S: 0% or more and 0.050% or less, 
 N: 0% or more and 0.010% or less, 
 O: 0% or more and 0.006% or less, 
 Mo: 0% or more and 1.000% or less, 
 Ti: 0% or more and 0.200% or less, 
 Nb: 0% or more and 0.200% or less, 
 B: 0% or more and 0.010% or less, 
 V: 0% or more and 0.200% or less, 
 Cu: 0% or more and 1.000% or less, 
 W: 0% or more and 0.100% or less, 
 Ta: 0% or more and 0.100% or less, 
 Ni: 0% or more and 1.000% or less, 
 Sn: 0% or more and 0.050% or less, 
 Co: 0% or more and 0.500% or less, 
 Sb: 0% or more and 0.050% or less, 
 As: 0% or more and 0.050% or less, 
 Mg: 0% or more and 0.050% or less, 
 Ca: 0% or more and 0.050% or less, 
 Y: 0% or more and 0.050% or less, 
 Zr: 0% or more and 0.050% or less, 
 La: 0% or more and 0.050% or less, 
 Ce: 0% or more and 0.050% or less, and 
 a remainder consisting of Fe and impurities, 
 wherein a metallographic structure at a position of ¼ of a sheet thickness from a surface includes, in volume percentage, 
 ferrite and epitaxial ferrite: 10% or more and less than 50%, 
 a proportion of the epitaxial ferrite in a total volume percentage of the ferrite and the epitaxial ferrite: 5% or more and 30% or less, 
 martensite: 20% or more and 70% or less, 
 bainite: 50% or less, 
 residual austenite: 15% or less, and 
 a remainder in microstructure: 5% or less, 
 a total volume percentage of the bainite, the residual austenite and the remainder in microstructure is 50% or less, 
 A/B, which is a ratio of a length A of an interface between the epitaxial ferrite and the ferrite to a length B of an interface between the epitaxial ferrite and the martensite in a cross section that is along a rolling direction and perpendicular to the surface at the position of ¼ of the sheet thickness from the surface, is more than 1.5, 
 a ratio of a major axis to a minor axis of the martensite is 5.0 or more, and 
 a tensile strength is 980 MPa or more. 
 
     
     
       2. The steel sheet according to  claim 1  further comprising, as the chemical composition, in mass %, one or more selected from the group of:
 Mo: 0.010% to 1.000%, 
 B: 0.0001% to 0.010%, 
 Ti: 0.010% to 0.200%, 
 Nb: 0.010% to 0.200%, 
 V: 0.010% to 0.200%, 
 Cu: 0.001% to 1.000%, and 
 Ni: 0.001% to 1.000%. 
 
     
     
       3. The steel sheet according to  claim 2 ,
 wherein a hot-dip galvanized layer is provided on the surface of the steel sheet. 
 
     
     
       4. The steel sheet according to  claim 2 ,
 wherein a hot-dip galvannealed layer is provided on the surface of the steel sheet. 
 
     
     
       5. The steel sheet according to  claim 2 ,
 wherein an electrogalvanized layer is provided on the surface of the steel sheet. 
 
     
     
       6. The steel sheet according to  claim 1 ,
 wherein a hot-dip galvanized layer is provided on the surface of the steel sheet. 
 
     
     
       7. The steel sheet according to  claim 1 ,
 wherein a hot-dip galvannealed layer is provided on the surface of the steel sheet. 
 
     
     
       8. The steel sheet according to  claim 1 ,
 wherein an electrogalvanized layer is provided on the surface of the steel sheet. 
 
     
     
       9. A manufacturing method of the steel sheet of  claim 1 , comprising:
 a hot rolling step of hot-rolling a slab having a chemical composition containing, in mass %, C: 0.050% or more and 0.500% or less, Si: 0.01% or more and 2.50% or less, Mn+Cr: 1.20% or more and 4.00% or less, Al: 0.10% or more and 2.00% or less, P: 0% or more and 0.100% or less, S: 0% or more and 0.050% or less, N: 0% or more and 0.010% or less, 0: 0% or more and 0.006% or less, Mo: 0% or more and 1.000% or less, Ti: 0% or more and 0.200% or less, Nb: 0% or more and 0.200% or less, B: 0% or more and 0.010% or less, V: 0% or more and 0.200% or less, Cu: 0% or more and 1.000% or less, W: 0% or more and 0.100% or less, Ta: 0% or more and 0.100% or less, Ni: 0% or more and 1.000% or less, Sn: 0% or more and 0.050% or less, Co: 0% or more and 0.500% or less, Sb: 0% or more and 0.050% or less, As: 0% or more and 0.050% or less, Ca: 0% or more and 0.050% or less, Y: 0% or more and 0.050% or less, Zr: 0% or more and 0.050% or less, La: 0% or more and 0.050% or less, Ce: 0% or more and 0.050% or less, and a remainder consisting of Fe and impurities, to produce a hot-rolled steel sheet in which a prior austenite grain size is smaller than 30 μm; 
 a cooling step of cooling the hot-rolled steel sheet to 500° C. or lower at an average cooling rate of 20° C./second or faster; 
 a coiling step of coiling the hot-rolled steel sheet after the cooling step at 500° C. or lower; 
 a cold rolling step of pickling the hot-rolled steel sheet after the coiling step and cold-rolling the hot-rolled steel sheet at a rolling reduction of 30% or smaller to produce a cold-rolled steel sheet; 
 an annealing step of heating the cold-rolled steel sheet to a first temperature range of (Ac3 point−100° C.) to 900° C. and soaking the cold-rolled steel sheet in the first temperature range for five seconds or longer; and 
 an annealing cooling step of cooling the cold-rolled steel sheet after the annealing step at an average cooling rate of 2.5° C./second to 50° C./second in a second temperature range of 750° C. to 550° C.; 
 thereby producing the steel sheet of  claim 1 . 
 
     
     
       10. The manufacturing method of the steel sheet according to  claim 9 ,
 wherein the hot rolling step includes a finish rolling step of rolling the slab by continuously passing the slab through a plurality of rolling stands, 
 wherein, in the finish rolling step, 
 a rolling start temperature at a third rolling stand from a last rolling stand is 800° C. to 1000° C., 
 in each of the last three rolling stands in the finish rolling step, the slab is rolled at a rolling reduction of larger than 10%, 
 an interpass time between the individual rolling stands in the last three rolling stands in the finish rolling step is 3.0 seconds or shorter, and 
 (T n −T n+1 ), which is a difference between an exiting-side temperature T n  of the n-th rolling stand of the last three rolling stands and an entering-side temperature T n+1  of the (n+1)-th rolling stand of the last three rolling stands in the finish rolling step, is larger than 10° C. 
 
     
     
       11. The manufacturing method of the steel sheet according to  claim 10 , wherein a hot-dip galvanizing layer is formed by immersing the cold-rolled steel sheet after the annealing cooling step in a hot-dip galvanizing bath. 
     
     
       12. The manufacturing method of the steel sheet according to  claim 11 , wherein the hot-dip galvanizing layer is alloyed in a temperature range of 300° C. to 600° C. 
     
     
       13. The manufacturing method of the steel sheet according to  claim 9 , wherein a hot-dip galvanizing layer is formed by immersing the cold-rolled steel sheet after the annealing cooling step in a hot-dip galvanizing bath. 
     
     
       14. The manufacturing method of the steel sheet according to  claim 13 , wherein the hot-dip galvanizing layer is alloyed in a temperature range of 300° C. to 600° C. 
     
     
       15. A steel sheet comprising, as a chemical composition, in mass %,
 C: 0.050% or more and 0.500% or less, 
 Si: 0.01% or more and 2.50% or less, 
 Mn+Cr: 1.20% or more and 4.00% or less, 
 Al: 0.10% or more and 2.00% or less, 
 P: 0% or more and 0.100% or less, 
 S: 0% or more and 0.050% or less, 
 N: 0% or more and 0.010% or less, 
 O: 0% or more and 0.006% or less, 
 Mo: 0% or more and 1.000% or less, 
 Ti: 0% or more and 0.200% or less, 
 Nb: 0% or more and 0.200% or less, 
 B: 0% or more and 0.010% or less, 
 V: 0% or more and 0.200% or less, 
 Cu: 0% or more and 1.000% or less, 
 W: 0% or more and 0.100% or less, 
 Ta: 0% or more and 0.100% or less, 
 Ni: 0% or more and 1.000% or less, 
 Sn: 0% or more and 0.050% or less, 
 Co: 0% or more and 0.500% or less, 
 Sb: 0% or more and 0.050% or less, 
 As: 0% or more and 0.050% or less, 
 Mg: 0% or more and 0.050% or less, 
 Ca: 0% or more and 0.050% or less, 
 Y: 0% or more and 0.050% or less, 
 Zr: 0% or more and 0.050% or less, 
 La: 0% or more and 0.050% or less, 
 Ce: 0% or more and 0.050% or less, and 
 a remainder comprising Fe and impurities, 
 wherein a metallographic structure at a position of ¼ of a sheet thickness from a surface includes, in volume percentage, 
 ferrite and epitaxial ferrite: 10% or more and less than 50%, 
 a proportion of the epitaxial ferrite in a total volume percentage of the ferrite and the epitaxial ferrite: 5% or more and 30% or less, 
 martensite: 20% or more and 70% or less, 
 bainite: 50% or less, 
 residual austenite: 15% or less, and 
 a remainder in microstructure: 5% or less, 
 a total volume percentage of the bainite, the residual austenite and the remainder in microstructure is 50% or less, 
 A/B, which is a ratio of a length A of an interface between the epitaxial ferrite and the ferrite to a length B of an interface between the epitaxial ferrite and the martensite in a cross section that is along a rolling direction and perpendicular to the surface at the position of ¼ of the sheet thickness from the surface, is more than 1.5, 
 a ratio of a major axis to a minor axis of the martensite is 5.0 or more, and 
 a tensile strength is 980 MPa or more. 
 
     
     
       16. A manufacturing method of the steel sheet of  claim 15 , comprising:
 a hot rolling step of hot-rolling a slab having a chemical composition containing, in mass %, 
 C: 0.050% or more and 0.500% or less, 
 Si: 0.01% or more and 2.50% or less, 
 Mn+Cr: 1.20% or more and 4.00% or less, 
 Al: 0.10% or more and 2.00% or less, 
 P: 0% or more and 0.100% or less, 
 S: 0% or more and 0.050% or less, 
 N: 0% or more and 0.010% or less, 
 O: 0% or more and 0.006% or less, 
 Mo: 0% or more and 1.000% or less, 
 Ti: 0% or more and 0.200% or less, 
 Nb: 0% or more and 0.200% or less, 
 B: 0% or more and 0.010% or less, 
 V: 0% or more and 0.200% or less, 
 Cu: 0% or more and 1.000% or less, 
 W: 0% or more and 0.100% or less, 
 Ta: 0% or more and 0.100% or less, 
 Sn: 0% or more and 0.050% or less, 
 Co: 0% or more and 0.500% or less, 
 Sb: 0% or more and 0.050% or less, 
 As: 0% or more and 0.050% or less, 
 Mg: 0% or more and 0.050% or less, 
 Ca: 0% or more and 0.050% or less, 
 Y: 0% or more and 0.050% or less, 
 Zr: 0% or more and 0.050% or less, 
 La: 0% or more and 0.050% or less, 
 Ce: 0% or more and 0.050% or less, and 
 a remainder comprising Fe and impurities, to produce a hot-rolled steel sheet in which a prior austenite grain size is smaller than 30 μm; 
 a cooling step of cooling the hot-rolled steel sheet to 500° C. or lower at an average cooling rate of 20° C./second or faster; 
 a coiling step of coiling the hot-rolled steel sheet after the cooling step at 500° C. or lower; 
 a cold rolling step of pickling the hot-rolled steel sheet after the coiling step and cold-rolling the hot-rolled steel sheet at a rolling reduction of 30% or smaller to produce a cold-rolled steel sheet; 
 an annealing step of heating the cold-rolled steel sheet to a first temperature range of (Ac3 point−100° C.) to 900° C. and soaking the cold-rolled steel sheet in the first temperature range for five seconds or longer; and 
 an annealing cooling step of cooling the cold-rolled steel sheet after the annealing step at an average cooling rate of 2.5° C./second to 50° C./second in a second temperature range of 750° C. to 550° C.; 
 thereby producing the steel sheet of  claim 15 .